1. Field of the Invention
The present invention relates to tissue holding devices, and more particularly to tissue holding devices and methods for making the same that are particularly suited for tissue reinforcement, approximation and/or repositioning, or for securing tissue to implantable prostheses.
2. Background Discussion
Sutures have been used for many decades for wound closure and/or tissue approximation in a variety of medical applications. More recently, barbed sutures have been gaining attention for various medical applications as well. Barbed sutures typically have a series of “barbs” that extend outwardly from the suture, with the objective being that such barbs increase the holding strength of the suture and/or eliminate the need for knot tying.
In most barbed sutures that exist in the marketplace, the barbs are formed by cutting into the suture shaft with a blade of some sort. An exemplary barbed suture 10 of this type is illustrated in FIG. 1. For each barb 11 that is cut into the suture shaft 12, the end of the cut area, or base 13 of the barb acts as a weak lever that allows the barb to bend backwards under stress. In addition, the diameter of the suture D1 is reduced along the area into which a barb has been cut (as illustrated at D2), which greatly reduces the tensile strength of the suture. Cutting into the suture shaft to form the barbs also has another disadvantage in that the number, size and geometry of the barbs is limited. This is so because the spacing between the barbs, the barb length etc. are greatly constrained by the size of the original suture because the cuts begin to interfere with one another and/or adversely affects the mechanical strength of the suture. There are also size limitations on what can be achieved with such a barbed suture. For example, as the barb size decreases, the size of the tooling necessary to cut the barbs becomes very small and precise, and thus very difficult to manufacture. Further, as the suture diameter decreases, cutting a barb into a very small cross-section decimates the strength of the suture due to the resultant thin core.
Other means for creating suture-like products have been suggested. For example, U.S. Patent Publication No. 2004/0060410 and WO 2006/005144 make reference to other processes such as injection molding, stamping and laser cutting. U.S. Patent Publication No. 2003/0149447 also suggests stamping, progressive die cutting, injection molding, and chemical etching as methods to produce barbed sutures out of a flat material. Each of these suggested methods has its drawbacks. For example, injection molding is a process by which liquid material is injected into a die until the material fills the die, and is then allowed to cool. The sample is subsequently ejected from the die. With this method, however, because the material is first melted to allow for injection into the die, any mechanical strength due to molecular orientation (i.e., such as that seen in a polymer that has been extruded) is mainly lost. Significant thermal treatment such as that experienced in injection molding, decreases the amount of molecular orientation as the chains are able to rearrange into a more random structure. The loss of molecular orientation can adversely affect mechanical properties such as yield strength and bending modulus, which adversely affects the holding strength of the suture. In addition, there is a limitation on the size and shape of the devices that can be made by injection molding. Capillary forces will limit the ability to fill a mold in small areas, long run areas, and intricate geometries. Also, injection molded parts suffer from issues such as warpage and shrinkage.
The publications mentioned above also suggest stamping and die cutting as alternative methods for forming such products. Die cutting is simply the process of cutting shapes from sheets by pressing a shaped knife-edge into one or more layers of sheeting. Stamping is a more general term to denote sheet material press-working. It typically involves impressing a material with some mark or figure. Because these processes only make imprints in, or “cookie cut” shapes in a given material, they cannot be used to create intricate 3-dimensional geometries. Further, die cutting or stamping processes do not promote material flow to fill in a double-female die cavity.
The present invention described herein provides new processes for forming tissue holding devices that overcome the disadvantages of the processes described above. Further, the present invention provides various tissue holding devices having unique geometries achievable using the methods described herein.